This invention relates to a direct current detection circuit for detecting a direct current value by using a zero-phase current transformer (hereinafter referred to as ZCT) making use of a magnetic material like as Permalloy, such as a direct grounding current detecting circuit for detecting the occurrence of a direct grounding current used, for example, for linkage protection of a linkage apparatus.
As an example of a direct grounding current detecting circuit serving as a direct current detecting circuit, Japanese Patent Publication Tokkai 11-122819 has disclosed the technology applicable to a linkage apparatus for converting direct-current power from a direct-current source into alternate-current power by using a current transformer to detect the difference between the current values flowing for the positive and negative terminals of the direct-current source and to judge according to the level of this current difference whether a direct grounding current is being generated.
With such a detection circuit, a direct grounding current accident of a system linkage apparatus can be prevented on the basis of the presence or absence of a direct grounding current because the occurrence of such a direct grounding current may be detected if the current difference detected by the current transformer exceeds a certain level while it is concluded that such a direct grounding current is absent if the detected current difference is found not to exceed this level.
The manner in which such a direct grounding current detecting circuit may be used in a linkage system is explained next with reference to FIG. 10 which is a block diagram showing the structure of a general linkage system 100 comprised of a system linkage apparatus 103 disposed between a direct current source 101 such as a solar battery and an alternate current system 102 such as a single-phase three-line type for converting the direct current power of the direct current source 101 into alternate current power. The system linkage apparatus 103 comprises an inverter 104 for converting the direct current power from the direct current source 101 into alternate current power, a linkage relay 105 for the ON/OFF control on the linkage with the alternate current system 102, a ZCT 10 for magnetically detecting the current difference between the U-phase source line 110A and the W-phase source line 110B between the inverter 104 and the alternate current system 102, a direct grounding current detection circuit 107 for detecting the presence or absence of a direct grounding current on the basis of the current difference detected by this ZCT 10 and a CPU 108 for controlling the inverter 104, the linkage relay 105, etc. on the basis of the detection result by the direct grounding current detection circuit 107.
The CPU 108 of the system linkage apparatus 103 serves to cut off the linkage with the alternate current system 102 by controlling the linkage relay 105 to be switched off when the direct grounding current detection circuit 107 detects the generation of a direct grounding current.
FIG. 11 is a block diagram for approximately showing the inner structure of the direct grounding current detection circuit 107. FIGS. 12A and 12B are waveform diagrams of various outputs for showing the operations of the direct grounding current detection circuit 107 respectively when it is judging that a direct grounding current is absent and present.
The direct grounding current detection circuit 107 comprises the aforementioned ZCT 10 having the U-phase source line 110A and the W-phase source line 110B inserted as explained above for magnetically detecting the current different therebetween and changing its self-impedance according to the change in the magnetic field generated by this current difference, an oscillator circuit 11 for generating a specified voltage V1, a voltage divider resistor 12 connected in series with the ZCT 10, a comparison voltage generating circuit 13 for generating a comparison voltage V5 on the basis of the voltage value V2 divided between the ZCT 10 and the voltage divider resistor 12 according to the change in the impedance of the ZCT 10 and a control circuit 14 for judging whether or not the comparison voltage V5 generated by the comparison voltage generating circuit 13 is above a threshold value for determining the presence or absence of a direct grounding current and detecting the presence or absence of the occurrence of a direct grounding current on the basis of the result of this judgment. In the above, the voltage value V2 is obtained as Z1/(Z1+R1)*V1 where Z1 is the impedance of the ZCT 10 and R1 is the resistance of the voltage divider resistor 12.
As shown in FIG. 11, the comparison voltage generating circuit 13 comprises a rectifier circuit 131 for detecting and rectifying the voltage value V2, an offset amplifier circuit 132 for offset-amplifying the output voltage V3 from this rectifier circuit 131 and a filter circuit 133 for filtering the output voltage V4 from this offset amplifier circuit 132 and thereby outputting a comparison voltage value V5.
The control circuit 14 judges that a direct grounding current has occurred relative to the source lines 110A and 110B if the comparison voltage value V5 from the comparison voltage generating circuit 13 is above a threshold value (a reference level) as shown in FIG. 12B, but that there is no occurrence of a direct grounding current if it is determined that the comparison voltage value V5 is not above the threshold value, as shown in FIG. 12A.
In summary, the prior art direct grounding current detection circuit 107 serves to generate the comparison voltage value V5 on the basis of the voltage value V2 divided by the ZCT 10 and the voltage divider resistor 12 according to the change in the impedance of the ZCT 10, determining the occurrence of a direct grounding current to be present or absence according to whether this comparison voltage value V5 is above or below the threshold value, and hence is capable of dependably detecting the presence or absence of a direct grounding current.
Although an example with an alternate current system with the single-phase three-line type was described above with reference to FIG. 10 with the U-phase and W-phase source lines 11A and 110B inserted through the ZCT 10, a similar effect can be obtained with a system of the single-phase two-line type by inserting the L-phase and N-phase source lines through the ZCT 10.
It is to be noted that the prior art direct grounding current detection circuit 107 employs a ZCT 10 made of Permalloy as its magnetic material for detecting the current difference between the source lines 110A and 110B. Since Permalloy has a hysteresis characteristic as shown in FIG. 13, the magnetic permeability-magnetization current characteristic of the ZCT 10 is as shown in FIG. 14. The prior art direct grounding current detection circuit 107 is therefore influenced by this hysteresis characteristic of its ZCT 10. If it is desired to detect the presence or absence of a direct grounding current of a very small value in the range of −30 mA to +30 mA (such as shown shaded in the graph of FIG. 15) such as 15 mA, the comparison voltage value V5 appears at two points and it becomes difficult to detect an accurate direct current value or to judge correctly if there is a direct grounding current.
This problem is not limited to the direct grounding current detection circuit 107. Direct current detection circuits are generally influenced by the hysteresis characteristic of their ZCT 10 and are not capable of accurately detecting a direct current value.